In a statement, S&P says that even though Berkshire Hathaway has an "excellent business profile," the lower credit rating "better reflects our view of BRK's dependence on its core insurance operations for most of its dividend income." (S&P's statement is posted on its website, but you have to register to view it.)

"Mr. Buffett has said he and Berkshire's board have decided who will follow him as chief executive, but they have chosen not to make it public. The company intends to split the chairman and chief executive role that Mr Buffett plays, with his son serving as non-executive chairman."

Buffett is 82.

The FT adds that "Mr. Buffett has a history of clashing with S&P. He criticized the agency in August 2011, when the credit rating agency downgraded the U.S.'s sovereign debt rating, saying it 'doesn't make sense.' A few days afterwards, S&P reduced its outlook on Berkshire Hathaway from 'stable' to 'negative.' "

As for what this all means, The Wall Street Journal's MoneyBeat blog says the downgrade "will have little effect" on Berkshire Hathaway's borrowing costs. And the news has not prompted any of the firms that deal with Berkshire Hathaway to demand that it put up more collateral, MoneyBeat reports.